Adjustment of ventilator pressure-time profile to balance comfort and effectiveness

ABSTRACT

The invention is a ventilator whose servo-controller adjusts the degree of support by adjusting the profile of the pressure waveform as well as the pressure modulation amplitude. As the servo-controller increases the degree of support by increasing the pressure modulation amplitude, it also generates a progressively more square, and therefore efficient, pressure waveform; when the servo-controller decreases the degree of support by decreasing the pressure modulation amplitude, it also generates a progressively more smooth and therefore comfortable pressure waveform.

FIELD OF THE INVENTION

[0001] This invention relates to the field of mechanical ventilation,and more particularly to machines and methods for providing a patientwith ventilatory support.

BACKGROUND

[0002] Conventional ventilators provide ventilatory support by utilizinga number of different pressure-time profiles. In its simplest form, aventilator delivers airflow at a fixed rate (or some other fixedfunction of time such as sinusoidally), and the airway pressureincreases passively as a function of the mechanical properties of thepatient's respiratory system. Such a ventilator is in general suitableonly for a paralyzed and sedated patient who cannot change his/herventilation at will. Also, the system is intolerant of leak, so isunsuitable for non-invasive (mask) ventilation.

[0003] A bilevel ventilator uses a square pressure-time waveform:

P=P _(0+A,) f>0

P=P₀, otherwise

[0004] where P₀ is an end expiratory pressure, chosen to splint theupper and lower airways and alveoli, A is a fixed pressure modulationamplitude chosen to supply a desired degree of support, and f isrespiratory airflow. Here, and throughout what follows, inspiratory flowis defined to be positive, and expiratory flow is defined to benegative. With bilevel support, the patient can breathe as much or aslittle as he wishes, by using greater or lesser effort, and the systemis somewhat less affected by leak. Some known ventilators, for example,the Servo 300 available from Siemens Medical, Iselin, N.J., and theVPAP-ST from ResMed, San Diego Calif., have an adjustment for changingthe initial rate of rise of pressure, with the intention of providing amore comfortable waveform by using a slower rate of rise. In such priorart, the clinician selects a particular waveform, but thereafter thewaveform does not change, and there is no automatic adjustment of thewaveform.

[0005] Moving on in complexity, a proportional assist ventilatorprovides pressure equal to an end expiratory pressure P₀ plus aresistance R multiplied by respiratory airflow, plus an elastance Emultiplied by the time integral of respiratory airflow:

P=P ₀ +Rf+E∫f dt, f>0

P=P ₀ +Rf, otherwise

[0006] (where the integral is from the time of start of the currentinspiration to the current moment) in which the resistance R is chosento unload some or all of the resistive work of breathing, and theelastance E is chosen to unload some or all of the elastic work ofbreathing (that is to say, the Rf term provides a pressure increment tooffset some or all of the effort required to get air to flow through themechanical passageways, and the integral term provides some or all ofthe pressure required to overcome the elastic recoil or springiness ofthe lungs and chest wall). A proportional assist ventilator amplifiespatient effort, delivering a natural-feeling waveform, and it is easierfor the patient to increase or decrease his ventilation than in the caseof bilevel support. However, a proportional assist ventilator isdisadvantageous for a patient with abnormal chemoreflexes, as inadequatesupport is provided during pathological reductions in effort such ascentral apneas and hypopneas.

[0007] Another approach is to provide a pressure-time profile that iscontinuous function of phase in the respiratory cycle:

P=P ₀ +AΠ(Φ),

[0008] where Π(Φ) is a waveform template function, for example, as shownin FIG. 1, and φ is the phase in the respiratory cycle. In FIG. 1, thewaveform template is a raised cosine during the inspiratory part of thecycle, followed by a quasi-exponential decay during the expiratoryportion. This shape will produce a quasi-normal and thereforecomfortable flow-time curve if applied to a passive patient with normallungs.

[0009] For example, a servo-ventilator can be constructed by setting thepressure modulation amplitude A to:

A=−G∫(0.5|f|−V _(TGT))dt,

[0010] where G is a servo gain (for example, 0.3 cmH₂O per L/min persecond), V_(TGT) is a desired target ventilation (e.g., 7.5 L/min), andthe integral is clipped to lie between A_(MIN) and A_(MAX) (for example,3 and 20 cmH₂O) chosen for comfort and safety. A servo-ventilator hasthe advantage of guaranteeing a specified ventilation. By settingA_(MIN) to be non-negative, the patient can at will comfortably breathemore than the target ventilation, but in the event of a failure ofcentral respiratory drive, the device will guarantee at least aventilation of V_(TGT).

[0011] Finally, the advantages of using a waveform template can becombined with resistive unloading:

P=P ₀ +Rf+AΠ(Φ),

[0012] where

A=−G∫(0.5|f|−V _(TGT))dt, 0<=A_(MIN)<=A<=A_(MAX)

[0013] as before, giving yet more comfort to an awake patient than inthe case previously considered, yet without losing a guaranteed minimumventilation of V_(TGT).

[0014] A disadvantage of the pressure waveform template shown in FIG. 1is that it is less efficient than a square wave. That is to say, for anygiven amplitude it provides less ventilatory support than a square wave.The waveform of FIG. 1 has only half the area of a square wave of thesame amplitude. This can be a problem in patients who require a veryhigh degree of support, or in the case of mouth leak, where in order toprovide a desired pressure modulation amplitude at the glottic inlet, amuch higher pressure modulation amplitude must be supplied at the mask.The use of pure resistive unloading is similarly inefficient for thesame reason: the area under the pressure-vs-time curve is only half thatof a square wave of the same amplitude. Even the combination of thesmooth waveform tempate with resistive unloading is less efficient thana square wave of the same amplitude.

[0015] It is a general object of our invention to provide a pressuresupport ventilator that offers the advantages of using a smooth pressurewaveform template while at the same time compensating for itsdisadvantages.

[0016] It is another object of our invention to balance comfort andeffectiveness in a ventilator.

SUMMARY OF THE INVENTION

[0017] One broad concept implemented by the invention is to change thepressure waveform in a way that makes an advantageous trade-off betweencomfort and efficiency, using a more efficient but less comfortablewaveform only when needed.

[0018] One aspect of the invention is a ventilator whoseservo-controller adjusts the degree of support by adjusting the profileof the pressure waveform, preferably while also adjusting the pressuremodulation amplitude.

[0019] In particular, the servo-controller increases the degree ofsupport by increasing the pressure modulation amplitude, and also bygenerating a progressively more square, and therefore efficient,pressure waveform; the servo-controller decreases the degree of supportby decreasing the pressure modulation amplitude, and by generating aprogressively more smooth and therefore comfortable pressure waveform.The changes in amplitude and squareness can be performed sequentially,or partially or completely simultaneously.

BRIEF DESCRIPTION OF THE FIGURES

[0020] Further objects, features and advantages of the invention willbecome apparent upon consideration of the following detailed descriptionin conjunction with the drawing, in which:

[0021]FIG. 1 depicts a smooth and comfortable ventilator waveformtemplate function;

[0022]FIG. 2 depicts illustrative apparatus for implementing the methodof the invention; and

[0023]FIGS. 3 and 4 show two alternative variable templates for use inthe apparatus of FIG. 2 in accordance with the invention, the shape ofeach template being a function of the instantaneous difficulty inventilating the patient.

DETAILED DESCRIPTION OF THE INVENTION

[0024] Suitable apparatus for implementing the invention is shown inFIG. 2. The apparatus provides breathable gas at controllable positivepressure to a patient's airway. In the drawing, a blower 10 suppliesbreathable gas to a mask 11 in communication with a patient's airway viaa delivery tube 12 and exhausted via an exhaust 13. Airflow at the mask11 is measured using a pneumotachograph 14 and a differential pressuretransducer 15. The mask flow signal f(t) from the transducer 15 is thensampled by a microprocessor 16. Mask pressure is measured at the port 17using a pressure transducer 18. The pressure signal from the transducer18 is then sampled by the microprocessor 16. The microprocessor sends aninstantaneous mask pressure request (i.e., desired mask pressure) signalP(t) to a servo-controller 19, which compares the pressure requestsignal with the actual pressure signal from the transducer 18 to controla fan motor 20. Microprocessor settings can be adjusted via a serialport, not shown.

[0025] It is to be understood that the mask could equally be replacedwith a tracheotomy tube, endotracheal tube, nasal pillows, or othermeans of making a sealed connection between the air delivery means andthe patient's airway.

[0026] The microprocessor accepts the mask airflow and pressure signals,and from these signals determines the instantaneous flow through anyleak between the mask and patient, by any convenient method. Forexample, the conductance of the leak may be estimated as theinstantaneous mask airflow, low-pass filtered with a time constant of 10seconds, divided by the similarly low-pass filtered square root of theinstantaneous mask pressure, and the instantaneous leakage flow may thenbe calculated as the conductance multiplied by the square root of theinstantaneous mask pressure. Respiratory airflow is then calculated asthe instantaneous mask airflow minus the instantaneous leakage flow.

[0027] Throughout the following discussion, the phase in the respiratorycycle Φ is taken as varing between zero and 1 revolution, with zerocorresponding to start of inspiration and 0.5 corresponding to start ofexpiration.

[0028] The desired mask pressure is described by the followingequations:

P=P ₀ +Rf+AΠ(Φ)

[0029] where:

[0030] P₀ is a desired end expiratory pressure chosen to splint theupper and lower airways or alveoli, or to reduce cardiac preload orafterload, for example, 5 cmH₂O;

[0031] R may be zero, but is preferably any value less than thepatient's actual airway resistance;

[0032] f is respiratory airflow, measured, for example, using apneumotachograph in the mask, and correcting for leak, for example, asdescribed in the commonly owned International Publication referred toabove;

[0033] Φ is the phase in the patient's respiratory cycle;

[0034] Π(Φ) is a pressure waveform template, initially set to be similarto that shown in FIG. 1, for example, comprising a raised cosinefollowed by an exponential decay.

[0035] In a very simple form, suitable for a patient who is making nospontaneous efforts, or in whom the spontaneous efforts can be ignored,the phase Φ simply increases linearly with time, modulo 1 revolution. Ina preferred form, the phase Φ is calculated, for example, from therespiratory airflow f using fuzzy logic as taught in the commonly ownedInternational Publication No. WO 98/12965 entitled “Assisted Ventilationto Match Patient Respiratory Need,” referred to above.

[0036] An example of a smooth and comfortable pressure waveform templateΠ(Φ) is shown in FIG. 1. This particular waveform consists of a raisedcosine followed by a quasi-exponential decay. (Unlike a true exponentialdecay, the waveform of FIG. 1 falls exactly to zero by the end ofexpiration, so that there is no step change at the start of the nextbreath.)

[0037] The first reason why the waveform of FIG. 1 is more comfortablethan a traditional square wave is that the more sudden changes inpressure associated with a square wave are more intrusive than thesmoother changes in pressure of FIG. 1.

[0038] The second reason why the waveform of FIG. 1 is more comfortable,whereas a traditional square wave is less comfortable, relates toprecise synchronization of the delivered pressure to the patient's ownmuscular efforts. The more precise the synchronization, the morecomfortable the waveform. The term R f in the equation

P=P ₀ +Rf+AΠ(Φ)

[0039] given above can be adjusted to obviate some or most of the effortrequired to unload resistive work. By a suitable choice of the amplitudeA, and a suitable waveform Π(Φ), the term A Π(Φ) can be adjusted tounload most of the normal or pathological elastic work at the eupneictidal volume, or alternatively at a minimum desired tidal volume,leaving the patient free to breathe deeper if he wishes. The reason forthis is that a eupneically breathing subject's inspiratory flow-timecurve is quasi-sinusoidal, and therefore the elastic component ofeffort, which is proportional to the integral of flow, is a raisedcosine. For this reason, the waveform of FIG. 1 has a raised cosineduring inspiration. During early expiration, a normal subject's musculareffort does not drop instantaneously to zero, but remains active sometime into expiration, decaying gradually, which maintains a high lungvolume longer than would otherwise be the case, thereby helping keep thealveoli inflated, and also providing a smoother movement of the chestwall. For this' reason; the waveform of FIG. 1 has a quasi-exponentialdecay during expiration. An additional advantage of the waveform of FIG.1 over a square wave is that small errors in timing of the start ofinspiration produce negligible errors in the delivered pressure, whereaswith a square wave, a timing error causes the delivered pressure to bewrong by the entire amplitude of the waveform. Therefore, the waveformof FIG. 1 will feel better synchronized to the subject's efforts than asquare wave.

[0040] Primary interest is in waveform templates which are nondecreasingduring the inspiratory half of the cycle, nonincreasing during theexpiratory half, and with the first derivative defined everywhere exceptat the transitions between inspiration and expiration and vice versa. Ofparticular importance are waveform templates which are families offunctions indexed by a single smoothness parameter K, which can forconcreteness take values between zero (least smooth, or most square) and1 (most smooth). The maximum absolute value of the derivative (of thewaveform template with respect to phase) increases as smoothness Kdecreases. Thus in the family of waveform templates shown in FIGS. 3 and4, each waveform is smoother than the waveform immediately to the left.

[0041] As the patient's ventilatory requirements increase, the smoothand comfortable waveform template changes to a progressively more square(and therefore more efficient, but generally less comfortable) waveform.In a preferred form, the pressure waveform template is a function of asmoothness variable K. When K=1.0, the template is smooth as shown inFIG. 1. When K=0.0, the template is a square wave, and intermediatevalues of K generate intermediate waveforms.

[0042]FIG. 3 shows one way in which the waveform can vary with K togenerate intermediate waveforms. During inspiration, Π(Φ) is a blendbetween a raised cosine and a rising exponential, the time constant ofthe exponential decreasing with K. During expiration, Π(Φ) is a decayingexponential, also with a time constant that decreases as K increases.Letting, for K>0

u=0.5[1−cos(2πΦ)]

v=a(1−e ^(−Φ/K))

[0043] where

a=1/(1−e ^(−2.5/K))

[0044] we define

Π(Φ)=Ku+(1−K)v, φ<0.5

Π(Φ)=1−a(1−e ^(−5(Φ−0.5)/K)) otherwise.

[0045] The equations degenerate to a square wave when K=0. The purposeof the constant a is to ensure that Π(Φ) approaches zero as Φ approaches0.5 and also as Φ approaches unity.

[0046] As K decreases, two things happen to the inspiratory part of thecurve: the exponential becomes progressively more like a rising stepfunction, and the exponential contributes progressively more to thetemplate, generating a family of curves intermediate between a raisedcosine and a square wave. Similarly, as K decreases, the exponential inthe expiratory part of the curve becomes more like a descending stepfunction.

[0047]FIG. 4 shows another method, in which the inspiratory part of thecurve is a raised cosine followed by a straight line:

Π(Φ)=0.5[1−cos(2πΦ/K)], φ<0.5, φ<0.5 K

Π(Φ)=1−a(1−e ^(−5(Φ−0.5)/K)), φ>0.5

Π(φ)=1, otherwise

[0048] where

a=1/(1−e ^(−2.5/K))

[0049] In this method, with K=1.0, the straight line segment vanishesand the inspiratory curve is a raised cosine. As K decreases, thestraight line segment lengthens and the raised cosine is squashedprogressively to the left. Again, the equations degenerate to a squarewave with K=0.0.

[0050] In both embodiments, the object is to use K=1.0 when smalldegrees of support are required, K=0.0 when very large degrees ofsupport are required, and intermediate values of K in between.

[0051] In a simple form of the invention, K is adjusted in order toservo-control the patient's minute ventilation to equal a chosen target.For example, K may be adjusted using clipped integral control asfollows:

K=G∫(0.5|f|−V _(TGT))dt, 0<=K<=1

[0052] where:

[0053] G is a gain, for example, 0.01 per L/min per second;

[0054] V_(TGT) is the chosen target ventilation, for example, 7.5 L/min;

[0055] The reason for dividing the absolute value of the respiratoryairflow by two is as follows. The target ventilation V_(TGT) isspecified with the units of L/min. Normally, ventilation is calculatedas either the entire volume inspired per minute (inspired minuteventilation), or the entire volume expired per minute (expired minuteventilation). Equally, it can be calculated as the average of these two,in which case the average minute ventilation is half the average of theabsolute value of the respiratory airflow over any given minute. Moregenerally, the average ventilation is the average of half the absolutevalue of the respiratory airflow over any chosen period of time.Omitting the averaging step, we see that the instantaneous ventilationis half the absolute value of the respiratory airflow, and the term 0.5|f|−V_(TGT) is the error in the instantaneous ventilation, and istherefore (on average) a measure of the adequacy of ventilation. If theterm 0.5 |f|−V_(TGT) is on average positive, then the subject requiresless ventilatory support, and conversely if it is on average negative,then the subject requires more ventilatory support. The clipped integralcontroller servo-controls this quantity to be zero on average, andtherefore servo-controls the instantaneous ventilation to on averageequal the target ventilation, whereupon the average ventilation alsoequals the target ventilation.

[0056] In this embodiment, if the subject is exceeding the targetventilation, the value of K will increase, yielding progressivelysmoother, more comfortable, but less efficient waveforms, until eitherthe actual ventilation decreases to equal the target ventilation, oruntil K reaches 1.0, which yields the smoothest waveform. Conversely, ifthe subject is not achieving the target ventilation, K will decreasegradually, causing the waveform to become more square and moreefficient, until either the target ventilation is achieved, or untilK=0.0, representing a perfectly square waveform. For example, if K=1.0,V_(TGT)=7.5 L/min, G=0.01 per L/min per second, and the subject ceasesall respiratory airflow, K will decrease to zero in 13.3 seconds.

[0057] There are two ways of increasing the degree of ventilatorysupport: using a more square waveform, and increasing the pressuremodulation amplitude A. Therefore, in the present invention, both thesmoothness K and the pressure modulation amplitude A may be adjusted,either simultaneously or sequentially, in order to achievesynergistically a desired target ventilation.

[0058] In a preferred form, a smooth waveform is used preferentially,and as far as possible the desired target ventilation is achieved bymodulating the amplitude A, but if this is unsuccessful, then aprogressively more square waveform is used, by decreasing K. Inaccordance with this form of the invention, the pressure modulationamplitude A may be adjusted using a clipped integral controller in orderto servo-control minute ventilation to equal a desired targetventilation as follows:

A=−G∫(0.5|f|−V _(TGT))dt, 0<=A_(MIN)<=A<=A_(MAX)

[0059] where:

[0060] G is a gain, for example, −0.3 cmH₂O per L/min per second;

[0061] V_(TGT) is a chosen guaranteed minimum (target) ventilation, forexample, 7.5 L/min;

[0062] A_(MIN) is a minimum pressure modulation amplitude, chosen tomake the patient comfortable while awake, for example, 3 cmH₂O; and

[0063] A_(MAX) is a maximum pressure modulation amplitude, chosen to besufficient to do all respiratory work, within the constraints oftolerability and safety, for example, 20 cmH₂O.

[0064] In the case where the patient's ventilation exceeds the targetV_(TGT), the pressure modulation amplitude A will reduce, until eitherthe ventilation on average equals V_(TGT) and A lies in the rangeA_(MIN)<A<A_(MAX), or until A reaches A_(MIN). Conversely, in the casewhere A_(MAX) is insufficient to ventilate the patient at V_(TGT), Awill become equal to A_(MAX).

[0065] In this preferred form, K is then calculated as a decreasingfunction of the pressure modulation amplitude A. In other words, as thepressure modulation amplitude A increases with the need for greaterventilatory support, K decreases to provide still further support (atthe expense of comfort). Therefore, the pressure waveform template Π(Φ)becomes a function of the pressure modulation amplitude A. The inventiondelivers a comfortable, smooth pressure-vs-phase (and thereforepressure-vs-time) curve, providing the target ventilation V_(TGT) isbeing achieved with a pressure modulation amplitude less than a chosenmaximum A_(MAX), but using a progressively more efficient, and thereforemore square, waveform in the case where the target ventilation cannot beachieved using the chosen maximum.

[0066] To this end, the smoothness K may be calculated using clippedintegral control using the following pseudocode: K = 1.0 REPEAT every 20milliseconds Calculate A IF A <A_(MAX) Increment K by 0.002 ELSEDecrement K by 0.002 END Truncate K to lie between 0.0 and 1.0 END

[0067] Initially, K=1.0, and the smoothest waveform will be used. In thecase where the patient is being well ventilated at or above the targetventilation V_(TGT), K will remain at 1.0 and the patient will continueto receive a very smooth and comfortable pressure waveform.

[0068] If the patient becomes difficult to ventilate, for example, dueto sputum retention, failure of respiratory drive, diaphragm fatigue,failure of accessory muscles of respiration, mouth leak, or a large leakwhich is exceeding the capacity of the blower, K will gradually decreasetowards zero.

[0069] The effect is that the actual delivered pressure waveform Π(Φ)changes gradually and continuously between the comfortable and smoothshape in FIG. 1, and a less comfortable but more efficient square wave.In the most severe case with ventilation remaining below V_(TGT) and Aremaining below A_(MAX), K will reach zero in about 10 seconds, and asquare wave will be delivered. In a less severe case, as K decreases andthe waveform becomes progressively more square, and therefore moreefficient at generating ventilation, V_(TGT) will be achieved at anintermediate value of K and therefore at an intermediate waveform shape.

[0070] Should the conditions which led to the requirement for a moreefficient waveform subside, the target ventilation V_(TGT) will be met,pressure modulation amplitude will reduce to below A_(MAX), and K willagain increase, yielding a smoother and more comfortable waveform.

[0071] In the example given above, K increases at a maximum rate of 0.1per second. Larger rates of change will produce a more rapid increase ineffectiveness of ventilatory support, but are likely to lead toovershoot, with oscillations in the degree of support. Smaller rates ofchange will be stable, but will take longer to re-establish ventilationat V_(TGT).

[0072] In the above algorithm, the ventilator attempts to cope with aneed for increased ventilatory support in two discrete stages, first byincreasing the pressure modulation amplitude, while maintaining thesmooth waveform, but only up to a preset maximum amplitude A_(MAX), andthen subsequently by using a progressively more efficient waveform. Inother embodiments it is possible for the two stages to overlap. Forexample, the pseudocode could be changed to: K = 1.0 REPEAT every 20milliseconds Calculate A Decrement K by 0.002(A − 0.75 A_(MAX)) TruncateK to lie between 0.0 and 1.0 END

[0073] This algorithm performs identically to the previous algorithm forthe extreme cases of a patient who is either very difficult or very easyto ventilate, but differs for intermediate cases, because the transitionfrom smooth to square begins earlier, at 75% of A_(MAX). If more than75% of the maximum pressure modulation is being used, K will decrease,and the waveform will become more square. Conversely, if more than 75%of the maximum pressure modulation is being used, K will increase andthe waveform will become more rounded. Thus when increasinglyventilating the patient, it is possible to adjust the trade-off betweenincreasing the pressure modulation amplitude and using a more efficientwaveform.

[0074] In some cases, it may be desirable to prevent K from reachingzero. For example, keeping 0.1<K<1.0 can produce almost as great anincrease in efficiency at low K, but is more comfortable to the patientthan a completely square waveform. This is particularly the case iflarge amounts of resistive unloading are used. (This is because anear-square waveform template on its own will produce a rapid increasein flow at start of inspiration, which will then produce yet furtherincrease in pressure due to resistive unloading.)

[0075] Alternatively, K can be made to increase quickly at first, andthen more slowly, so that the most square waveform is used only as alast resort, for example, by submitting K to a square root or similartransform. In other cases, with patients with considerable air hungerand intrinsic PEEP, it may be desirable to limit K to a value less than1.0, although in general it would be preferable to increase theresistive unloading R and the end expiratory pressure P₀. In the aboveembodiments, K is related to the integral of A (minus a threshold) withrespect to time, essentially using an integral controller to determineK, in an attempt to servo-control ventilation to equal or exceedV_(TGT). In other embodiments, other known controllers such as PIDcontrollers may be used.

[0076] Although the invention has been described with reference toparticular embodiments, it is to be understood that these embodimentsare merely illustrative of the application of the principles of theinvention. In the above preferred embodiments, the pressure waveform isa function of phase in the respiratory cycle φ, calculated as as taughtin the commonly owned International Publication No. WO 98/12965 entitled“Assisted Ventilation to Match Patient Respiratory Need”. However, if itis not desired to synchronize with the patient's spontaneous efforts,phase can be taken as increasing linearly with time at a preset rate,modulo 1 revolution. In this manner, the pressure waveform is a simplefunction of time, and the invention simplifies to modifying the shape ofa fixed pressure-vs-time waveform. Thus, the pressure waveform may be afunction of the phase in the patient's respiratory cycle, or time, or ofboth. Similarly, in the above preferred embodiments, linear resistiveunloading is used, but the invention is applicable in the case of noresistive unloading, and also in the case of nonlinear resistiveunloading. In the preferred embodiments described above, the pressurewaveform template comprises a raised cosine followed by aquasi-exponential decay. However, the precise waveform is not overlycritical. Waveforms with the broad general features of FIG. 1 aresatisfactory, and will generally produce large improvements in comfortand synchronization over a square wave. The waveform may be modified tomore or less precisely include eupneic resistive unloading by settingthe waveform to be more or less closely the shape of the subject'seupneic transdiaphragmatic pressure vs phase curve. Again, in thepreferred embodiments, a fixed non-zero end expiratory pressure is used,yet the invention extrapolates naturally to either zero end expiratorypressure or to automatically adjusted end expiratory pressure.Similarly, some specific examples of how to adjust the shape of thepressure waveform are given, but these are intended only as examples. Inthe examples given, both the inspiratory and expiratory phases of thepressure waveform template increase or decrease their smoothness. In theillustrative embodiments, a single parameter K defines the shape of thepressure waveform template. Also contemplated are embodiments in whichmore than one parameter defines the template. One example is to use oneparameter for inspiration and another for expiration, and vary themindependently. Another is to use one parameter which chiefly affectsearly inspiration and expiration, and another which chiefly affects lateinspiration and expiration. Although the specific implementationdelivers air from a blower, the invention works equally well with air,oxygen, or other breathable gases, and any source of breathable gas atcontrollable pressure may be used. Numerous other modifications may bemade in the illustrative embodiments of the invention and otherarrangements may be devised without departing from the spirit and scopeof the invention.

What we claim is:
 1. A method for controlling a pressure supportventilator comprising the steps of: supplying breathable gas to apatient's airway at a pressure that varies during each respiratory cyclein accordance with an adjustable pressure waveform template, determiningthe adequacy of the patient's ventilation, and automatically changingthe shape of said pressure waveform in accordance with said adequacy,wherein said change in shape produces an increase in ventilatory supportat the expense of comfort when the ventilation is inadequate, andproduces a more comfortable waveform at the expense of a decrease inventilatory support when ventilation is excessive.
 2. A method inaccordance with claim 1 wherein the pressure of the gas supplied to thepatient's airway is also a function of an amplitude factor that varieswith adequacy of the patient's ventilation, the amplitude factor and thewaveform shape changing in directions that synergistically change thepatient's ventilation.
 3. A method in accordance with claim 2 whereinairflow is servo-controlled to adjust said amplitude factor so thatventilation equals a target ventilation, and the shape of said waveformis controlled in accordance with said amplitude factor.
 4. A method inaccordance with claim 3 wherein said waveform varies between a waveformchosen to be particularly efficient when maximum ventilatory support isrequired, and a waveform chosen to be particularly comfortable when lessventilatory support is required.
 5. A method in accordance with claim 4wherein said efficient waveform is a square wave.
 6. A method inaccordance with claim 4 wherein said comfortable waveform is a smoothwaveform chosen to generate an approximately normal flow versus timecurve.
 7. A method in accordance with claim 1 wherein ventilation isservo-controlled so that ventilation equals a target value, and theshape of said waveform is controlled to be a function of respiratoryairflow.
 8. A method for controlling a patient ventilator comprising thesteps of: supplying air to a patient's airway at a pressure that variesduring each respiratory cycle in accordance with an adjustable pressurewaveform template, repeatedly determining from the supplied airflow theadequacy of the patient's ventilation, and changing said adjustablepressure waveform template in a predetermined manner in accordance withthe determined adequacy of ventilation, the changes controlling anincrease in ventilatory support at the expense of comfort when moresupport is required and controlling a decrease in ventilatory supportfor the greater comfort of the patient when less support is required. 9.A method in accordance with claim 8 wherein the pressure supplied to thepatient's airway is in part a function of an amplitude factor thatvaries with the ventilatory requirements of the patient, the amplitudefactor and the waveform shape changing in directions thatsynergistically change the patient's ventilation.
 10. A method inaccordance with claim 9 wherein the patient's ventilation isservo-controlled by adjusting said amplitude factor so that airflowequals a target value, and said waveform template is controlled to be afunction of said amplitude factor.
 11. A method in accordance with claim10 wherein said waveform varies between a waveform chosen to beparticularly efficient when maximum ventilatory support is required, anda waveform chosen to be particularly comfortable when less ventilatorysupport is required.
 12. A method in accordance with claim 11 whereinsaid efficient waveform is a square wave.
 13. A method in accordancewith claim 11 wherein said comfortable waveform is a smooth waveformchosen to generate an approximately normal flow versus time curve.
 14. Amethod in accordance with claim 8 wherein ventilation isservo-controlled so that ventilation equals a target value, and theshape of said waveform is controlled to be a function of respiratoryairflow.
 15. A method for controlling a pressure support servoventilator comprising the steps of: supplying air to a patient's airwayat a pressure that varies during each respiratory cycle, the pressurebeing characterized by both a pressure modulation amplitude and anadjustable pressure waveform, and controlling both said pressuremodulation amplitude and the profile of said adjustable pressurewaveform by the output of a servo-controller operated to servo-controlpatient ventilation to at least equal a target value.
 16. A method as inclaim 15 in which said servo-controller increases the degree of supportby at least one of increasing the pressure modulation amplitude andgenerating a progressively more square pressure waveform.
 17. A methodas in claim 15 in which said servo-controller decreases the degree ofsupport by at least one of decreasing the pressure modulation amplitudeand generating a progressively more smooth pressure waveform.
 18. Amethod as in claim 15 in which the adjustable pressure waveform isinfinitely adjustable between a chosen smoothest profile and a chosenmost square profile.
 19. A method as in claim 15 in which the adjustablepressure waveform is adjusted to be smoother if said pressure modulationamplitude is less than a threshold, and adjusted to be more squareotherwise.
 20. A method for controlling a pressure support ventilatorcomprising the steps of: supplying air to a patient's airway at apressure that varies during each respiratory cycle, the pressure beingcharacterized by both a pressure modulation amplitude and an adjustablepressure waveform of adjustable shape, deriving a measure ofinstantaneous patient ventilation, and controlling both said pressuremodulation amplitude and the shape of said adjustable pressure waveformin accordance with said measure of patient ventilation.
 21. A method asin claim 20 in which the degree of ventilatory support is increased byat least one of increasing the pressure modulation amplitude andgenerating a progressively more square pressure waveform.
 22. A methodas in claim 20 in which the degree of ventilatory support is decreasedby at least one of decreasing the pressure modulation amplitude andgenerating a progressively more smooth pressure waveform.
 23. A methodas in claim 20 in which the adjustable pressure waveform is infinitelyadjustable between a chosen smoothest shape and a chosen most squareshape.
 24. A method as in claim 20 in which said adjustable pressurewaveform is adjusted to be smoother if said pressure modulationamplitude is less than a threshold, and adjusted to be more squareotherwise.
 25. A pressure support ventilator comprising a supply of airfor a patient's airway whose pressure varies during each respiratorycycle in accordance with an adjustable pressure-versus-time waveform, acontroller for determining from the supplied airflow the patient'sventilatory requirements, and means for changing the shape of saidpressure-versus-time waveform in accordance with the determinedventilatory requirements, the waveform controlling an increase inventilatory support at the expense of comfort when more support isrequired and controlling a decrease in ventilatory support for thegreater comfort of the patient when less support is required.
 26. Apressure support ventilator in accordance with claim 25 wherein the airsupplied to the patient's airway is also a function of an amplitudefactor that varies with the ventilatory requirements of the patient, andover at least a portion of each of successive respiratory cycles theamplitude factor and the waveform shape change in directions that causean airflow change in the same direction.
 27. A pressure supportventilator in accordance with claim 26 wherein airflow isservo-controlled to adjust said amplitude factor so that airflow equalsa target value, and the shape of said waveform is controlled to be afunction of said amplitude factor.
 28. A pressure support ventilator inaccordance with claim 27 wherein during the inspiratory portion thereofsaid waveform varies between an approximately square wave for largeventilatory support but less comfort and a smooth shape thatapproximates a normal breathing pattern for less ventilatory support butgreater comfort.
 29. A pressure support ventilator in accordance withclaim 27 wherein airflow is continuously sampled and the waveform iscontinuously changed in accordance with the sampled values.
 30. Apressure support ventilator in accordance with claim 25 wherein airflowis servo-controlled so that airflow equals a target value, and the shapeof said waveform is controlled to be a function of airflow.
 31. Apressure support ventilator in accordance with claim 25 wherein duringthe inspiratory portion thereof said waveform varies between anapproximately square wave for large ventilatory support but less comfortand a smooth shape that approximates a normal breathing pattern for lessventilatory support but greater comfort.
 32. A pressure supportventilator comprising a supply of air for a patient's airway whosepressure varies during each respiratory cycle as a predeterminedfunction of time, a controller for continuously determining from thesupplied airflow the patient's ventilatory requirements, and means forcontinuously changing said predetermined function of time in apredetermined manner in accordance with the determined ventilatoryrequirements, the changes controlling an increase in ventilatory supportat the expense of comfort when more support is required and controllinga decrease in ventilatory support for the greater comfort of the patientwhen less support is required.
 33. A pressure support ventilator inaccordance with claim 32 wherein the air supplied to the patient'sairway is in part a function of an amplitude factor that varies with theventilatory requirements of the patient, and over at least a portion ofeach of successive respiratory cycles the amplitude factor and thepredetermined function of time change in directions that cause anairflow change in the same direction.
 34. A pressure support ventilatorin accordance with claim 33 wherein airflow is servo-controlled toadjust said amplitude factor so that airflow equals a target value, andthe continuous changes in said predetermined function of time arecontrolled to be a function of said amplitude factor.
 35. A pressuresupport ventilator in accordance with claim 34 wherein during theinspiratory portion thereof said predetermined function of time variesbetween an approximately square wave for large ventilatory support butless comfort and a smooth shape that approximates a normal breathingpattern for less ventilatory support but greater comfort.
 36. A pressuresupport ventilator in accordance with claim 34 wherein airflow iscontinuously sampled and the predetermined function of time iscontinuously changed in accordance with the sampled values.
 37. Apressure support ventilator in accordance with claim 32 wherein airflowis servo-controlled so that airflow equals a target value, and thecontinuous changes in said predetermined function of time are controlledto be a function of airflow.
 38. A pressure support ventilator inaccordance with claim 32 wherein during the inspiratory portion thereofsaid predetermined function of time varies between an approximatelysquare wave for large ventilatory support but less comfort and a smoothshape that approximates a normal breathing pattern for less ventilatorysupport but greater comfort.
 39. A pressure support ventilatorcomprising a supply of air for a patient's airway whose pressure variesduring each respiratory cycle, the pressure being characterized by botha pressure modulation amplitude and an adjustable pressure waveform, andmeans for controlling both said pressure modulation amplitude and theprofile of said adjustable pressure waveform by the output of aservo-controller operable to servo-control patient ventilation to atleast equal a target.
 40. A pressure support ventilator as in claim 39in which said servo-controller increases the degree of support by atleast one of increasing the pressure modulation amplitude and generatinga progressively more square pressure waveform.
 41. A pressure supportventilator as in claim 39 in which said servo-controller decreases thedegree of support by at least one of decreasing the pressure modulationamplitude and generating a progressively more smooth pressure waveform.42. A pressure support ventilator as in claim 39 in which the adjustablepressure waveform is infinitely adjustable between a chosen smoothestprofile and a chosen squarest profile.
 43. A pressure support ventilatoras in claim 39 in which said adjustable pressure waveform is adjusted tobe smoother if said pressure modulation amplitude is less than athreshold, and adjusted to be more square otherwise.
 44. A pressuresupport ventilator comprising a supply of air for a patient's airwaywhose pressure varies during each respiratory cycle, the pressure beingcharacterized by both a pressure modulation amplitude and an adjustablepressure waveform, means for deriving a measure of instantaneous patientventilation, and means for controlling both said pressure modulationamplitude and the profile of said adjustable pressure waveform inaccordance with said measure of patient ventilation.
 45. A pressuresupport ventilator as in claim 44 in which the degree of ventilatorysupport is increased by at least one of increasing the pressuremodulation amplitude and generating a progressively more square pressurewaveform.
 46. A pressure support ventilator as in claim 44 in which thedegree of ventilatory support is decreased by at least one of decreasingthe pressure modulation amplitude and generating a progressively moresmooth pressure waveform.
 47. A pressure support ventilator as in claim44 in which the adjustable pressure waveform is infinitely adjustablebetween a chosen smoothest profile and a chosen most square profile. 48.A pressure support ventilator as in claim 44 in which said adjustablepressure waveform is adjusted to be smoother if said pressure modulationamplitude is less than a threshold, and adjusted to be more squareotherwise.
 49. A method for providing positive pressure ventilationcomprising the steps of: supplying a patient with breathable gas at apressure which is a function of at least the phase in the respiratorycycle, an amplitude, and a smoothness parameter; determining thesubject's ventilation; and adjusting said smoothness parameter within apreset range to servo-control said ventilation to at least equal atarget ventilation.
 50. A method as in claim 49, in which said amplitudeis adjusted within a preset range to servo-control said ventilation toat least equal a target ventilation.
 51. A method as in claim 50, inwhich said smoothness parameter is calculated from said amplitude.
 52. Amethod as in claim 51, in which said servo-control is performed firstlyby increasing or decreasing said amplitude, and secondarily adjustingsaid smoothness parameter if said amplitude is not within a specifiedrange.
 53. A pressure support ventilator comprising a supply of air fora patient's airway whose pressure is a function of at least a measure ofthe phase in the patient's respiratory cycle, an amplitude, and asmoothness parameter.
 54. A pressure support ventilator as in claim 53,in which said amplitude is adjusted within a preset range toservo-control said ventilation to at least equal a target ventilation.55. A pressure support ventilator as in claim 54, in which saidsmoothness parameter is calculated from said amplitude.
 56. A pressuresupport ventilator as in claim 54, in which said servo-control isperformed firstly by increasing or decreasing said amplitude, andsecondarily adjusting said smoothness parameter if said amplitude is notwithin a specified range.